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A two machine power system is shown below. Transmission line XY has positive sequence impedance of Z1 Ω and zero sequence impedance of Z0 Ω.
An ‘a’ phase to ground fault with zero fault impedance occurs at the centre of the transmission line. Bus voltage at X and line current from X to F for the phase ‘a’, are given by Va Volts and Ia Amperes, respectively. Then, the impedance measured by the ground distance relay located at the terminal X of line XY will be given by
- Z1/2 Ω
- Z0/2 Ω
- (Z0+Z1)/2 Ω
- Va/Ia Ω
Correct answer: Va/Ia Ω
Solution
The impedance measured by the ground distance relay is calculated using the voltage at the relay location divided by the current flowing through the fault, which is represented by Va/Ia. This relationship holds true for any fault condition, making this option the correct choice.
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